This study develops a portable direct shear apparatus to test the in-situ strength of fine-grained soil.The instrument features a modular design，integrating an in-situ sampling system，vertical hydraulic propulsion system，vertical loading system，horizontal tensile shear system，automated measurement and control system，and a modular mobile platform.This configuration enables automated in-situ sampling and comprehensive shear testing of undisturbed soil specimens at varying depths.Laboratory tests on fine sand and clay validated the instrument's feasibility and accuracy.The results demonstrated deviations are less than 5%in shear strength parameters compared to conventional direct shear apparatus，confirming strong data consistency.A linear relationship between stress ratio and strain increment gradient was observed prior to peak strength，aligning with the stress-dilatancy equation proposed by Matsuoka，thereby verifying the reliability of the instrument.